The combination of a metal salt of pyrithione and cuprous oxide is known as an excellent biocide composition in antifouling paint formulations. However, paints containing commercial grades of metal salts of pyrithione such as zinc pyrithione and cuprous oxide have been known to thicken, agglomerate or gel during formulation and storage of the paint. Such agglomeration or gelation results in poor storage stability and decreased shelf life of the in-can paint or paint base formulation.
Specifically, it has been noted when certain hydrolysable acrylate resins are present in paint formulations with zinc pyrithione and cuprous oxide, viscosity increases and agglomeration and gelation become problematic during the formulation and storage of the paint. Hydrolysable acrylate resins such copper, zinc and silyl acrylate resins are commonly used in antifouling paint and paint base formulations.
Hydrolysable acrylate resins are known to gradually undergo hydrolysis underwater to form a coating film on a surface as described in U.S. Pat. Nos. 5,080,892; 6,458,878; 5,332,431 and 4,898,895.
The cause and mechanism of the instability problems associated with antifouling paints containing hydrolysable acrylate resins and the combination of zinc pyrithione and cuprous oxide have not been clearly understood. Suggested causes include low quality of zinc pyrithione with high content of impurities and agglomerated particles; low quality of cuprous oxide with higher content of copper metal and copper ions; or chemical reaction of zinc pyrithione with cuprous oxide.
Several patents have disclosed various approaches in an effort to address the gelation problem that exists in paints containing zinc pyrithione in combination with cuprous oxide. For example, U.S. Pat. No. 5,112,397 discloses a process for providing a gelation-inhibited paint, which employs an organic amine or esterified wood rosin in order to impart the desired gelation-inhibition to the paint. However, this process is dependent on ingredients such as polymer resin. Additionally, the organic amines are detrimental from an environmental standpoint.
International Patent Application WO 1992/015647 describes a process for stabilizing zinc pyrithione and cuprous oxide in paint using high purity zinc pyrithione and an amine compound or esterified wood rosin. EP 610251 describes a process for stabilizing paint using 1-20% of a metal salt of rosin. However, the stabilization achieved in both processes is limited. Additionally, hydrolysable acrylate resins are not easily usable in the paints disclosed in these references.
U.S. Pat. Nos. 5,098,473 and 5,137,569 disclose stable compositions that resist gelation. For instance, U.S. Pat. No. 5,098,473 utilizes purified zinc pyrithione to provide gelation-inhibition in paint. U.S. Pat. No. 5,137,569 adds a metal salt of wood rosin wherein the metals salts is selecting from calcium, zinc, copper, iron, nickel, magnesium salts, and their combinations thereof to the paint to provide gellation-inhibition of the paint. However, the storage stability of the coatings disclosed in these references works best at lower or moderate temperatures and therefore would not perform well, or at all, in higher temperature climates.
Recently-published U.S. patent application 2005/123503 discloses a storage stable composition containing a hydrolysable acrylate resin, cuprous oxide, and also a 2-hydroxypyridine-N-oxide stabilizer, or the sodium, zinc or copper salt thereof. However, the storage stability of these pyridine-N-oxide-containing stabilizers in hydrolysable acrylate resin-based paint is less than might be desired.
Accordingly, the storage instability of a paint composition containing a metal salt of pyrithione, cuprous oxide and hydrolysable acrylate resins remains a problem in the art. The present invention is believed to provide one such solution.